To avoid having to be reduced to such an eventuality it has already been thought of to apply to cracked structures the injection technique which has been employed for a long time for consolidating soils and terrain. In fact, by injecting under pressure into the cracks a product which is sufficiently fluid to enable it to enter within the structure, it is possible to fill the internal cracks and cavities so as to fill them in and even to consolidate the structure if the injected product exhibits good adhesiveness and strength properties.
Thermoplastic or heat-curable resins, such as polymers, which offer a very wide viscosity range enabling them to enter very fine cracks, have been employed hitherto in most cases to repair concrete structures by injection. However, such products exhibit disadvantages. First of all, their cost is high and makes a large-scale intervention very expensive. Above all, however, their elasticity modulus after curing is very different from that of concrete, and this creates hard spots which may be detrimental to the whole. In addition, their expansion coefficient is also different from that of concrete, and this presents an obvious disadvantage in the case of structures which are generally situated in the open air and are consequently subject to large temperature variations.
Attempts have therefore been made to employ other products and, in particular, those which were already being employed for consolidating and waterproofing soils. For example, patent application GB-2,195,138 describes an apparatus and a process which can be employed for injecting a curable product into cracks, it being possible for the injected product to be a polyester or epoxy resin or else a cement grout. However, the injection of cracks with a width of less than 2 mm is not envisaged.
As a general rule, the use of cement-based grouts to consolidate concrete structures is particularly advantageous since the injected product is, if not identical, in any case similar to that of the structure to be repaired and therefore exhibits an elasticity modulus and an expansion coefficient of the same order.
A major difficulty is encountered, however, as soon as the width of the crack or of the cavity opening becomes quite small. In practice, when commercial cements are employed, it is found that, if the width of the crack is less than 3 mm, a dewatered cement plug is formed as soon as the grout has entered the first centimeters, making it possible to pursue the operation.
To measure the performance of an injection product a standardized test has been developed, known as "sand column injectability test in dry and wet environment", forming the subject of NF Standard P 18-891. In such a test the time taken by the product being tested to reach the different marked lines placed along a transparent plastic column filled with calibrated sand is measured. The injectability properties of various cement grouts can thus be determined.
Such tests, carried out systematically on different products, have made it possible to ascertain that a cement grout could be injected only into holes or interstices whose width is at least 1.5 to 2.3 times the size of the largest cement particles.
Experience has shown, however, that, even when the size of the largest particles is restricted to the mentioned value, there was no hope of injecting cracks of less than 1 mm with a cement grout.
Now, in many cases it would be advantageous to be able to consolidate much finer cracks. It is obviously preferable, in fact, to intervene as soon as it is noticed that cracks have formed, before they widen. Furthermore, if the work is to be carried out on a road or on a structure in the open air, it is better to do this in the summer or in the spring, that is to say at a time when, because of the expansions, the cracks have closed up. In fact, the cracks are wider in winter, but at that time the days are shorter and, above all, there are risks of frost, especially at night.